Method of making a permanent-magnetisable body of compressed fine particles of a compound of m and r

ABSTRACT

A method of manufacturing a permanent magnet from compressed M5R powder (M is, for example, Co, R is a rare earth or Th). The powder is obtained by grinding castings in oxygen- and water-free environments in an inert gas or an inert liquid. The resultant magnet exhibits a small ageing effect, while in addition magnets of higher density can be obtained.

Unite States Westendorp et al. [451 May 16, 1972 [54] METHOD OF MAKING APERMANENT- [56] References Cited MAGNETISABLE BODY OF UNITED STATESPATENTS COMPRESSED FINE PARTICLES OF A 3 523 836 8/1970 B h 1 148/103 Xusc ow eta. COMPOUND OF M AND R 3,540,945 1 1/1970 Stmat et al. [72]Inventors: Frans Frederik westendorp; Antonius 3,463,678 8/ 1969 Becker..l48/ 105 Gregorius Riinbeek, both of Emm asingel, Eindhoven,Netherlands Primary Examiner-John F. Campbell Assistant Examiner--Ca.rlE. Hall [73] Asslgneez U.S. Philips Corporation, New York, NY. Anomeyprank R Trifari [22] Filed: Nov. 18, 1970 [57] ABSTRACT 21 A l.N 90,5121 pp 0 A method of manufacturing a permanent magnet from compressed M Rpowder (M is, for example, Co, R is a rare earth [30] ForeignApplication Priority Data or Th). The powder is obtained by grindingcastings in oxygenand water-free environments in an inert gas or aninert liquid. Dec. 20, Netherlands The resultant magnet exhibits a Smallg g effect, in

addition magnets of higher density can be obtained. [52] US. Cl..l48/l03, 29/208,148/105,

264/24, 264/ l l l 1 Claim, 2 Drawing Figures [51] Int. Cl. ..H0lf 7/06[58] Field of Search ..29/608; 252/6257, 62.56;

264/24, 111; 335/296; l48/l0l, 103, 105

Pakented May 16, 1972 3,663,317

2 Sheets-Sheet 6 ume) INVliN'l'URfi FRANS F. WESTENDORP ANTONIUS G.RIJNBEEK 1/ (C v k Va Paimt ed ay 16, 1972 3,6331? .2 Sheets-Sheet 2INVEN'I'ORS FRANS F. WESTENDORP ANTONIUS G. RUNBEEK AG /v 7 METHOD OFMAKING A PERMANENT-MAGNETISABLE BODY OF COMPRESSED FINE PARTICLES OF ACOMPOUND OF M AND R The invention relates to a method of manufacturing apermanent-magnetisable body of fine particles of a compound of M and R,the range of existence of which is integral with that of the compound MR having a hexagonal crystal structure, in which M represents Co or acombination of Co with one or more of the elements Fe, Ni and Cu and Rrepresents one or more of the elements of the rare earth metals and/orTh, in which method a casting of the alloy concerned of M and R isground to a powder of the desired granular size, the powder beingpre-densified and orientated simultaneously or subsequently in amagnetic field, after which it is compressed by an isostatic pressure ofat least kbar and finally plastically deformed slightly, while saidisostatic pressure is maintained.

In this connection the rare earths are to include also the element Y.

The term range of existence is to denote herein a range ofconcentrations within which for a continuous series of compounds thesame crystal structure can be obtained for 100 percent.

Such a method is described in Journal of Applied Physics" Vol. 40, No.l0, pages 4,029 and ff. The resultant magnetic body described thereinmay have a relative density of 97 percent at the most, which is to beunderstood to mean that the density of the body is 97 percent of thetheoretical density of the M-R compound concerned.

Magnetic bodies thus manufactured have, however, the disadvantageousproperty that the coercive force decreases with time: there occurs aso-called ageing process. In the aforesaid publication this ageing isreferred to and recognized as a disadvantage to practical use of themagnetic bodies. This ageing increases with increasing temperature.However, even at room temperature ageing occurs to an undesirableextent.

In Cobalt 36 (1967), page 40 the cause of ageing is said to be oxidationin air. Since a magnetic body of the kind set forth, having a density of95 percent or more does not exhibit a porosity such that oxygen or watervapour can penetrate into the body, oxidation is impossible except onthe surface. Nevertheless such magnetic bodies are found to age to anappreciable extent.

It was a surprise to find that when the aforesaid known method ofmanufacturing a magnetic body is carried out on powder obtained, inaccordance with the characteristic aspect of the method according to theinvention, by grinding and thereafter pressing and orienting it inoxygen and water-free environments in an inert gas or in an inertliquid, magnetic bodies can be obtained, which exhibit ageing to aconsiderably lesser extent than the bodies hitherto known.

Said advantageous effect of said step during the grinding process onageing, is the more surprising as, when considering oxidation in air asbeing the cause of ageing, it cannot be accounted for how any oxidation,which occurs only for a short time i.e. during the grinding process, maybe the cause of the ageing effect in a later stage i.e. when themagnetic body has been manufactured.

The method according to the invention will be described more fully withreference to the following example.

Castings of the compound SmCo were ground by means of a coffee mill in aso-called glove box (gas-tight space in which protrude two glovesfastened by the sleeves in holes of the wall and being thus accessiblefrom without) into powder of an average granular size of about 3 pm. Theglove box was filled with He gas containing less than 5 ppm of oxygenand/or water vapour. The resultant powder, still in the glove box, waspoured into a lead holder, orientated in a magnetic field andpre-densified by an isostatic pressure of 8 kbar. Then the lead holderwith the block resulting from the compression, was

taken out of the glove box and subjected to the known compressionmethod: an isostatic pressure of more that l0 kbar during which aslight, plastic deformation was brought about with the block retainingits anisotropic characteristics.

The resultant permanent-magnetisable body was found to have a relativedensity of 97 percent. The body was then exposed to a temperature of Cin air. After 10 minutes the coercive force was found to be 15.900 Oe.After l00, 1,000 and 10,000 minutes the coercive forces were againmeasured. Graph (a) of FIG. 1 illustrates the variation of the coerciveforce as a function of time. It should be noted that the ageing effectoccurs about 2,000 times more rapidly at a temperature of 100 C than atroom temperature. Apart from the time associated with a temperature of100 C the time associated with a temperature of 27 is therefore plottedon the abscissa.

For illustrating the surprising, favourable effect of the method inaccordance with the invention, the graph of FIG. 1 also shows an ageingcurve (b) measured on a permanentmagnetisable body having also a densityof 97 percent and made from a powder ground by the same method andhaving the same chemical composition and the same average granular size,however, not being ground under the conditions characteristic of themethod according to the invention.

Apart from the aforesaid advantageous effect of the method according tothe invention on the ageing process of the final magnetic body a secondsurprising effect is found to occur: when the castings are ground inaccordance with the invention it is found to be possible to manufactureby said known compression method magnetic bodies having a relativelydensity exceeding the 97 percent, which hitherto was attainable at themost.

Measurements of ageing have shown that such magnetic bodies exhibit anageing effect decreasing in accordance with an increasing density.

The graph of FIG. 2 illustrates for a few magnetic bodies therelationship between the coercive force at the time t(H (t)) and thecoercive force at the time t 10 minutes (H,( 10)) and the time duringwhich the magnetic bodies were exposed to air at 100 C. The magneticbodies having densities of 98.5 to 99.9 percent were manufactured by themethod according to the invention, whereas the bodies having densitiesof 95.0 and 97.0 percent were manufactured by the method hitherto known.

An additional advantage of permanent magnets manufactured from bodieshaving such high densities resides in that the energy product (BH),,,,,as compared with that of permanent magnets formed from magnetic bodiesdiffering only in density therefrom, is higher.

The invention relates furthermore to permanent-magnetisable bodiesmanufactured by the method according to the invention and particularlyto such bodies having a relative density of more than 99 percent.

What is claimed is:

A method of manufacturing a permanent-magnetisable body formed fromfine. particles of a compound of M and R, the range of existence ofwhich is consistent with that of the compound M R having a hexagonalcrystal structure, wherein M represents Co or a combination of Co withone or more of the elements Fe, Ni, and Cu and wherein R represents oneor more of the elements of the rare earth metals and/or Th in which acasting of an alloy of M and R is ground to a powder of the desiredgranular size, said powder being pre-densified and oriented in amagnetic field, after which it is compressed by an isostatic pressure ofat least 10 kbar and finally deformed to a slight extent in a plasticmanner while retaining its anisotropic characteristics while saidisostatic pressure is maintained, characterized in that the grinding andthereafter the predensifying and orienting is performed in an oxygenandwater-free environment in an inert fluid.

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